A Second-Order Dual Mode Repetitive Control for Magnetically Suspended Rotor

In magnetically suspended rotor (MSR) systems, harmonic current mainly originates from rotor mass imbalance and sensor runout, which can generate multifrequency vibration. Repetitive control (RC) is an effective method to eliminate periodic disturbance from a control system. Conventional RC (CRC) method requires the exact knowledge of the period of the periodic signal. However, in some applications, the period may fluctuate or may not be available with sufficient accuracy, in which case the performance of the CRC severely decays. Besides, the transient response of the CRC is relatively slow. In response to these issues, a second-order RC that incorporates dual mode structure scheme is presented in this paper. By introducing Lagrange-interpolation-based fractional delay filters, the current suppression accuracy gets enhanced in fractional cases. The stability criterion with rigorous proof for the proposed scheme is addressed. The sensitivity to period-time of the proposed scheme is analyzed as well, from the aspect of sensitivity transfer function. Simulation and experimental validations are performed on an MSR to show the effectiveness of the proposed approach.